Permeation Properties

41.2 Permeation Properties

Permeation defines the ease with which fluids, both liquids and gases, can enter into, or move through concrete. The ability of fluids to enter into concrete is sometimes refers to as penetrability of concrete. Three fluids are relevant to durability and they are water, carbon dioxide and oxygen. Water can be detrimental in either its pure (uncontaminated) form or if it is contaminated with aggressive ions such as chlorides and sulfates.

Concrete is a porous medium with permeation properties controlled by the microstructure of its hardened cement paste, which in turn, is determined by the cementitious materials (CM) used, the water to cementitious material ratio (W/CM), the paste volume, and the extent of curing and compaction. Within this cement paste, the transition zone, i.e., the interface between the cement paste and the aggregate, is known to be more porous than the bulk of the cement paste. Thus, it is the microstructure of this transition zone that controls the permeation of concrete. As far as the ease of movement of fluids through concrete is concerned, three transport mechanisms should be distinguished and they are per- meability, diffusion and sorption.

Permeability refers to the flow of water through concrete under a pressure differential. The rate of flow follows Darcy’s law for laminar flow through a porous medium. It depends on the pressure gradient and size of interconnected pores in the cement paste. For flow to occur, the concrete has to be in its saturated conditions with relevant pores being continuous and greater than 120 nm. Quantitatively, this property is discussed in terms of the coefficient of permeability, commonly expressed in meters per second (m/s). Permeability is a relevant property to be measured in assessing the durability and service- ability of structures like dams, foundations, and underground structures, where they are in constant contact with water.

Diffusion is the process whereby gases (e.g., carbon dioxide or oxygen) or ions in solution (e.g., chlorides) enter concrete under a differential in concentration. The diffusion of these species can be described by Fick’s law. Diffusivity or diffusion coefficient, in m 2 /s, is often used to refer to the rate at which these species entering concrete. In addition to concentration gradient and sizes of capillary pores, the rate of diffusion is influenced by the type of penetrating species and the chemical properties of the concrete. Diffusion of gases is very slow in saturated concrete and is, therefore, a property relevant to concrete in aboveground structures such as buildings and bridges, where concrete is partially dry. For the durability of submerged or underground structures, the diffusion of chloride and sulfate ions should

be considered. Sorption or absorption is a result of capillary movement of liquids in the pores of the hardened cement paste under ambient conditions. Note that capillary suction occurs in dry or partially dry concrete, a condition commonly occurred in practice for aboveground structures. Sorption is relevant, particularly to coastal structures, where chloride salts carried by wind deposit on concrete surfaces. Once wetted by rain, water carrying chloride ions is absorbed into the concrete. The rate at which liquids, mainly water,

absorbed into concrete is often referred to as sorptivity or absorptivity, in m/s 0.5 . This parameter is highly dependent on the initial moisture content of the concrete and therefore, the test method used.

To achieve good quality concrete in practice with low permeation properties, engineers should specify concrete with low water to cementitious materials ratio, adequate initial curing and proper compaction. In testing for permeation properties, it is important to recognize the type of structure under consideration and its service environment. This helps to identify the transport mechanism and the appropriate per- meation property to be measured.